There is known a solar power generation system which assembles a plurality of solar cells to configure a solar cell panel and supplies the electric power generated by that solar cell panel to a load.
A solar cell fluctuates in output power according to the irradiance of the incident sunlight or the ambient temperature. The maximum power operation point also greatly fluctuates. For this reason, a maximum power point tracking (hereinafter abbreviated as MPPT) control method of detecting the maximum output power of a solar cell panel, which fluctuates according to the irradiance or other conditions, has been proposed.
Further, in a grid interconnection type solar power generation system in which a solar cell panel is connected to a load, there is proposed MPPT control using the so-called “hill-climbing method” of searching for the maximum output power of the solar cell panel (for example, PLT 1, Japanese Patent Publication No. 7-234733 A1).
In the above MPPT control, in order to search for the maximum power point of the solar cell panel with a high precision, it is sufficient that the control processing system reduce the amount of change of the width of the pulse which is applied to a gate of a switching element in a DC-DC converter used as the power converting means. However, in order to reduce the amount of change of the pulse width, it becomes necessary to raise the switching frequency of the DC-DC converter or suitably improve the processing capability of the control processing system. For realization of that, there are restrictions in practical use in terms of technology, price, etc.
For example, if reducing the amount of change of the pulse width, a longer time is taken for finding the maximum power point in response to a change of the irradiance of the light irradiated to the solar cell panel and therefore the response becomes a problem. There was therefore a limit to raising the efficiency of power conversion.
On the other hand, if increasing the amount of change of the width of the pulse which is applied to the gate of the switching element in order to improve the response, the power greatly fluctuates in the steady state, therefore the efficiency of power acquisition falls.
The inventors of the present application came up with an invention which improves the control method already in practical use in grid interconnection (for example PLT 2, Japanese Patent No. 4294346).
Below, the invention disclosed in PLT 2 will be summarized.
In the maximum power detection mode, the system controls the switching element configuring the DC-DC converter to change the output current of the solar cell panel which flows to an inductor configuring the DC-DC converter from zero to a short-circuiting current and thereby instantaneously scan a current-voltage (I-V) characteristic of the solar cell panel.
Then, in the tracking operation mode, the system controls the switching element in the DC-DC converter using the detected output current of the solar cell panel as the optimal solar cell current so as to track the output current of the solar cell panel using this optimal solar cell current as the reference signal and operates the panel at the obtained maximum power operation point.
By alternately performing the operation in the maximum power detection mode of detecting the maximum power and the operation in the tracking operation mode of performing an operation for tracking the optimal solar cell current detected in this way and by repeatedly operating in cycles of the maximum power detection mode and tracking operation mode, the maximum power point is reliably and strictly found, and power output control can be performed which tracks the optimal solar cell current in accordance with the maximum power point found.